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Bimodal Volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and Tectonic Implications of Age-Progressive Rhyolites

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Bimodal Volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and Tectonic Implications of Age-Progressive Rhyolites Article Volume 14, Number 8 8 August 2013 doi: 10.1002/ggge.20175 ISSN: 1525-2027 Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites Mark T. Ford, Anita L. Grunder, and Robert A. Duncan College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon, 97331, USA ([email protected] or [email protected]) [1] Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age-progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern-most Basin and Range (NWBR) rhyolite volcanism into a single age-progressive trend. The spatial-temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment-filled basins within the more volcanically active HLP. The west- northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi-stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR. Components: 14,332 words, 7 figures, 2 tables. Keywords: high lava plains; bimodal volcanism; intraplate volcanism; Ar-Ar geochronology; Oregon geology; slab rollback. Index Terms: 1115 Radioisotope geochronology: Geochronology; 1033 Intra-plate processes: Geochemistry; 1031 Sub- duction zone processes: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plate proc- esses: Mineralogy and Petrology; 8170 Subduction zone processes: Tectonophysics; 3060 Subduction zone processes: Marine Geology and Geophysics; 8415 Intra-plate processes: Volcanology; 8413 Subduction zone processes: Volcanology. Received 30 January 2013; Revised 6 May 2013; Accepted 14 May 2013; Published 8 August 2013. Ford, M. T., A. L. Grunder, and R. A. Duncan (2013), Bimodal volcanism of the High Lava Plains and northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites, Geochem. Geophys. Geosyst., 14, 2836–2857, doi:10.1002/ggge.20175. © 2013. American Geophysical Union. All Rights Reserved. 2836 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175 1. Introduction this tectonically complicated region, and contain multiple episodes of Oligocene and younger volca- [2] The late Cenozoic tectonomagmatic evolution nism. Over the past 12 Ma, silicic volcanism has of the Pacific Northwestern United States is com- produced a WNW age-progressive trend across the plex, involving three geodynamic mantle features: HLP-NWBR, nearly opposite from the trend of co- the Yellowstone hotspot, mantle processes related eval age-progressive rhyolites of the Yellowstone- to the subduction of the Cascadia slab, and exten- Snake River Plain system [MacLeod et al., 1976; sion of the Basin and Range. These tectonic- Jordan et al., 2004]. In contrast, basaltic volca- magmatic elements contribute to the formation of nism over the same period has not left a systematic the largest Cenozoic continental volcanic province temporal pattern in either the HLP-NWBR or in the world, including the Columbia River Yellowstone-Snake River Plain provinces [cf., Basalts, Steens Basalts, volcanic provinces of the Kuntz et al., 1992; Jordan et al., 2004]. Most Yellowstone—Snake River Plain track, Owyhee workers attribute the HLP age-progressive rhyo- Plateau and High Lava Plains (HLP), and lites to lithospheric processes such as extension northwestern-most Basin and Range (NWBR) [Fitton et al., 1991; Christiansen et al., 2002] or (Figure 1) [Smith and Luedke, 1984; Pierce and extension coupled with crustal block rotation Morgan, 1992; Hooper et al., 2002; Shoemaker [Carlson and Hart, 1987] or to a westward spread and Hart, 2002]. This region is a modern example of a portion of the deep-seated Yellowstone plume of intracontinental magmatic modification of the head, with or without involvement of Cascadia lithosphere. slab subduction [Draper, 1991; Camp and Ross, [3] The HLP and NWBR physiographic provinces 2004; Jordan et al., 2004]. Here we report 18 new of Oregon (Figure 1) are prominent elements of 40Ar/39Ar age determinations for rhyolitic Figure 1. Regional tectonic setting of the High Lava Plains (outlined in red) and surrounding provinces. The Northwest Basin and Range refers to that portion of the Great Basin that extends into southern Oregon. Dashed line indicates approximate position of the 87Sr/86Sr 0.706 line [after Kistler and Peterson, 1978], representing the western edge of the Proterozoic or older craton of North America with generally Mesozoic and younger accreted terranes to the west. Lightly shaded region indicates middle Mio- cene and younger extensional normal faulting (Basin and Range style) [after Pezzopane and Weldon, 1993]. Two sets of arrows give the absolute plate motions and velocities based on the NUVEL-1A model of DeMets, et al. [1994] and the HS3-NUVEL-1A model of Gripp and Gordon [2002]. North American Plate motion is between 19 and 26 km/Ma in central Oregon, near the center of the studied area (outlined by blue dashed box) and the Juan de Fuca Plate ranges from 16 to 19 km/Ma. Physiographic provin- ces adapted after Dicken [1950]; Walker and MacLeod [1991]; MacLeod, et al. [1995]; Shoemaker and Hart, [2002]. 2837 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175 Figure 2. Slopeshade DEM and fault map of south-central and southeastern Oregon. Major, generally NNE trending Basin and Range type faults are shown with thick lines. Thin, generally NW trending lines obliquely cutting the HLP represent the Brothers Fault Zone while the NWBR is similarly cut but the Eugene—Denio Fault Zone. Heavy red dashed line outlines the HLP physio- graphic province as described in the text and Figure 1. The HLP has generally muted terrain (lighter shading) compared to sur- rounding provinces. Figure is adapted from the Oregon State Geologic Map [Walker and MacLeod, 1991] and simplified from the spatial database of Miller et al. [2003]. samples, mostly from the NWBR, and further concentrated study (e.g., Glass Buttes). The recent document the distribution of westward age- geophysical and geological investigation of the progressive rhyolites within Oregon. We also pro- HLP has sparked a comprehensive body of work, vide a comprehensive, quantitative volume esti- much of which is distilled in this Geochemistry mates for volcanic units in this bimodal province Geophysics Geosystems volume. which has erupted nearly equal volumes of basalt- basaltic andesite and rhyolite over the past 12 Ma. [5] The physiographic HLP (Figure 1) generally Cascadia slab plate-driven asthenospheric flow is comprises a late Miocene and younger volcanic the driving force of our model for volcanism in the province that is approximately 90 km wide and HLP and NWBR and no plume is required. 275 km long extending across eastern and central Oregon. It is bounded by the Owyhee Plateau, Cascades volcanic arc, extending Basin and Range 1.1. Physiographic and Tectonic (the NWBR) and to the north by the relatively Background of the HLP-NWBR unextended, accreted terranes of the Blue Moun- [4] Since the original reconnaissance mapping sur- tains region. Generally, late Miocene and younger veys of Russell [1884, 1905], there has been per- flat-lying basaltic lava flows, ignimbrites and vol- sistent interest in the igneous rocks and tectonics caniclastic sediments cover much of the muted to- of the HLP-NWBR region [e.g., Williams, 1935; pography that characterizes the HLP (Figure 2). Green, 1973; Wells, 1979; Carlson and Hart, Oligocene through middle Miocene silicic vol- 1987; Draper, 1991; Johnson and Ciancanelli, canic rocks are also sparsely exposed within the HLP [MacLeod et al., 1976; McKee and Walker, 1984; Johnson and Grunder, 2000; Jordan et al., 1976; Jordan et al., 2004] (Figures 3a and 3b). 2004; Boschmann, 2012]. Much of the work over the past three decades is contained in university [6] The HLP is cut obliquely by the Brothers Fault student theses with a few specific areas receiving Zone, which is characterized by small, northwest- 2838 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175 Figure 3. Map of the distribution of central and eastern Oregon rhyolites and basalts shows the spatial distribution of silicic lavas and tuffs though time with (a) John Day age (37–20 Ma), (b) Steens age (17.5–15 Ma), (c) Deschutes age (7–6 Ma), and (d) age- progressive HLP and NWBR <12 Ma
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